Ok, I'm not really certain that it fails with all changes of database. But I do know that I had a problem importing into PostgresQL what I exported from HSQL. I don't think it's a problem with communicating to PostgresQL because it generates all the tables.

Here's a high-level log:

set up with HSQL
enter data
export
shutdown
set up again with HSQL
import fine
set up again with PostgreSQL
import hangs

Maybe this issue is really about a problem with my Confluence talking to my PostgresQL database. If so then I think there's an issue here with verifying the connection immediately after it's configured.